Tuesday, February 26, 2019

How Deadly Are Dead Zones: A Better Way to Know Found

Measuring the volume of dead zones in the Gulf of Mexico may be a better way to determine their deadliness. 

Dubravko Justic, Texaco Distinguished Professor in the LSU’s Department of Oceanography and Coastal Sciences, or DOCS, and Lixia Wang, Research Associate IV in DOCS, recently co-authored a study suggesting that measuring the volume rather than the area of the Gulf of Mexico’s “Dead Zone,” is more appropriate for monitoring its effects on marine organisms.

The “Dead Zone,” a hypoxic zone, is a region of low oxygen that results from high-nutrient runoff, such as nitrogen and phosphorus often found in fertilizer, flowing from the Mississippi River into the coastal ocean. It is the largest recurring hypoxic zone in the United States, occurring most summers, and is located off the coast of Louisiana. This nutrient pollution, coupled with other factors, is believed to have a negative impact on fisheries because it depletes the oxygen required to support most marine life in bottom and near-bottom waters.

Figure caption: Midsummer (July 21 – 26) hypoxic area (left) and hypoxic volume (right) in the northern Gulf of Mexico hypoxic zone simulated by the Justic and Wang (2014) model for present day condition (2002, upper), for a 25% N reduction scenario (middle) and under a 50% N reduction scenario (lower). Color bars on the left denote bottom dissolved oxygen concentrations (mg/l); color bars on the right denote the thickness of the hypoxic layer (m). The solid black line in the upper left plot denotes the areal extent of hypoxia (dissolved oxygen < 2 mg/l) in bottom waters observed during the July 21 – 26, 2002, hypoxia monitoring cruise. 
Photo Credit: Dubravko Justic

Since 2001, stakeholders have used hypoxic area measurements to set goals for limiting or reversing its size, but this new study shows that the hypoxic volume appears more responsive to reductions in nitrogen flowing into the northern Gulf of Mexico than the hypoxic area. The researchers’ model simulations indicate that even under a modest 25% nitrogen load reduction, the thickness of the hypoxic layer in the northern Gulf of Mexico decreases markedly, and hypoxia remains localized to a relatively thin layer near the bottom that most fish and other mobile organisms can more effectively avoid.

Justic believes this should be considered when reviewing and potentially setting new hypoxia management goals.

“Understanding variability in hypoxic volume is relevant to assessing the effects of hypoxia on fish and fisheries, such as enhanced susceptibility to fishing due to an increased aggregation of fish avoiding hypoxic waters,” Justic said.

LSU is at the forefront of coastal research and education with more than 200 faculty exploring and collaborating on every continent. This study was published in the journal Environmental Research Letters and included co-authors from the University of Michigan, North Carolina State University and NOAA Southeast Fisheries Science Center, in addition to Louisiana State University’s College of the Coast & Environment. The full study is available here: https://iopscience.iop.org/article/10.1088/1748-9326/aaf938/meta.

Contacts and sources:
 Christine Wendling
LSU College of the Coast & Environment

Alison SatakeLSU

Citation:Hypoxic volume is more responsive than hypoxic area to nutrient load reductions in the northern Gulf of Mexico—and it matters to fish and fisheries. Donald Scavia, Dubravko Justić, Daniel R Obenour, J Kevin Craig and Lixia Wang  Environmental Research Letters, Volume 14, Number 2 https://iopscience.iop.org/article/10.1088/1748-9326/aaf938/meta

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